Devesh Vyas, MD
Excimer laser phototherapeutic keratectomy (PTK) was approved by the U.S. Food and Drug Administration (FDA) in 1995 for the treatment of recurrent corneal erosions, irregular astigmatism, and elevated epithelial or anterior stromal opacities caused by trauma, infection, or corneal dystrophies.1 During PTK, a 193-nm argon-fluoride laser is used to photoablate the affected areas and create a smooth transparent surface. Compared to manual lamellar techniques, PTK results in controlled deposition of new extracellular matrix and basement membrane, reepithelialization, and stromal remodeling.2
Ideal candidates for PTK are patients with significant visual compromise, pathology in the anterior one-third of the cornea, an elevated or flat opacity, and recurrent erosions not responsive to medical therapy. Additional indications include postrefractive surgery, stromal haze, and intraepithelial dysplasia. Relative contraindications include pathology deeper than one-third depth, thinner corneas with risk of posterior corneal ectasia (usually if the residual central thickness is less than 250 mcg), and active ocular or adnexal infection or inflammation. Patients with inactive herpetic corneal scarring may require perioperative acyclovir for 2 to 6 weeks.3
Prior to surgery, complete medical and ocular histories are obtained and a full ophthalmic examination is performed. Testing includes best-corrected visual acuity, pinhole acuity, rigid gas-permeable contact lens over-refraction, glare testing, pupil diameter in different illuminations, videokeratography, pachymetry, and, in cases of recurrent erosions, gentle probing of loose epithelium. Informed consent is obtained after a discussion of risks, benefits, and alternatives.
Preoperatively, topical anesthesia and standard sterile preparation and technique are applied. Excimer ablation, repetition rates, and fluences are usually standardized according to the laser type and vary according to surgeon preference. Usually, the corneal epithelium is debrided but, in some instances, transepithelial ablation with the laser is preferable. Subepithelial scars are removed manually if possible, but in patients with significant surface irregularity caused by anterior stromal lesions, it may be best to ablate directly through the corneal epithelium to take advantage of its natural smoothing or masking ability. Intact epithelium autofluoresces a light purple hue during laser beam interaction, but this hue changes to a dark purple-black when ablation begins to break through the epithelial layer. Ideally, the breakthrough spots correspond to the elevated areas of pathology visualized at the slit lamp preoperatively.
When an excimer laser beam encounters an irregular corneal surface profile, it will ablate and remove corneal tissue, but the pattern of the irregularity is preserved and etched into deeper corneal layers. Masking agents are variable viscosity solutions of carboxymethylcellulose or similar substances that can be applied to an irregular corneal surface and act to regulate the peaks and valleys so that the excimer laser beam encounters a smooth corneal surface.4 The excimer beam can simultaneously ablate the corneal tissue peaks and the masking agent that fills the valleys so that the peaks are leveled, but the valleys are prevented from deepening.
The end point of the PTK session requires judgment. If the opacity is clear and the cornea is smooth, the session is finished. If the opacity is clear and the cornea still slightly rough, a final polish with carboxymethylcellulose is often used. An opacity may not be totally removable but partial removal may be sufficient to markedly improve the visual acuity.
Postoperative regimens after PTK include immediate administration of topical antibiotics and topical nonsteroidal agents. A disposable bandage soft contact lens is placed. Oral pain medications are prescribed as needed and the patient is cautioned to minimize activity. The patient is seen on postoperative day 1 and every 2 to 3 days until the epithelial defect heals, at which time the bandage soft contact lens is removed.
For patients with deeper treatments, postoperative reticular haze is commonly seen; thus, a course of topical steroids is prescribed starting at four times a day and tapered according to response.4 In the majority of patients, epithelialization is complete by 1 week postoperatively. The next postoperative visit is required at 1 month. Final refractions are done no sooner than the 1-month visit to allow for wound healing to progress. Stromal remodeling, changes in epithelial thickness, and astigmatism shifts can affect the final refractive result.
In 1988, investigational clinical trials of PTK involving human subjects began under the guidance of the FDA and two laser manufacturers - Summit Technology (Waltham, MA) and VISX (Sunnyvale, CA).1 The Summit study consisted of 13 centers distributed across the country and 249 PTK-treated eyes. All patients were older than 21 years and were placed into one of three treatment groups - corneal scars, irregular astigmatism, or recurrent corneal erosion. The VISX study consisted of 17 investigational sites with 271 patients. It included four patient groups - corneal opacities, irregular corneal surface, superficial infectious keratitis resistant to medical therapy, and postsurgical refractive abnormalities. The clinical trials proved the efficacy and safety of PTK, with both lasers receiving FDA approval in 1995.
In the Summit study, 70% to 77% of patients in the corneal opacity and irregular surface groups had substantial visual improvement, although 18% to 22% patients experienced loss of at least one line of vision. Most of the patients in the recurrent erosion group had no change in vision, which was expected as the goal of surgery was to relieve pain from epithelial breakdown rather than to improve vision. However, only 8% of the recurrent erosion group had any loss of acuity after the procedure.
Corneal cylinder reductions were seen after PTK most significantly in the irregular astigmatism group, which was also expected as the goal of PTK in this group is corneal smoothing. The scar patients had some reduction in cylinder with laser surface polishing, whereas there was little effect on cylinder in the recurrent erosion group.
The success of PTK is largely measured by improvement in best-corrected visual acuity. Maloney and colleagues reported 45% of their patients had an improvement in best-corrected spectacle acuity at 12 and 24 months, while 9% and 8% of treated eyes lost two or more lines of vision at 12 and 24 months, respectively. 5 Sher and associates reported similar results, with 48% of their patients showing improvement in vision by two or more lines after PTK and 15% showing worsening by two or more lines. 6 Azar and colleagues reported that the average improvement in best-corrected visual acuity was 1.8 lines. 7 In this study, 10% of patients lost two or more lines of best-corrected visual acuity while 45% gained two or more lines. Other studies confirmed these results.
The outcome of PTK also depends on the underlying corneal pathology. Maloney and colleagues showed that treatment was most effective in eyes with hereditary corneal dystrophies, Salzmann's nodular degeneration, or corneal scars. 5 Eyes with calcific band keratopathy appeared to improve the least. The visual outcome of treating corneal scars most likely depends on the stromal depth of the scar. Generally, patients with more superficial corneal pathology (anterior 100 mcg), regardless of the underlying cause, tend to have better outcomes. Postoperative infections are rare.
Changes in refractive error are common following PTK. Postoperatively induced hyperopia is directly related to the number of stromal pulses. Maloney and colleagues reported 41% and 48% of patients had a hyperopic shift of 1 D or more at 12 and 24 months, respectively. 5 Previous studies reported hyperopic shifts in 50% to 81% of treated eyes.8,9 Treatment of deeper stromal lesions can be expected to produce more significant postoperative hyperopia. Regular astigmatism may change following PTK, with 30% to 50% of treated eyes developing an increase in astigmatism of 1 D or more.6,9 Stability of the mean refraction and the mean best spectacle-corrected visual acuity can occur at the third postoperative month.
A recent advance in PTK is the development of collagen polymer gels that are applied to a rough corneal surface followed by a hard contact lens of a predetermined curvature placed atop the gel.10 When the gel polymerizes to its more solid state, the contact lens is removed and the smooth curvature of the lens is transferred to the corneal surface. Since the gel is made of collagen similar to corneal collagen, it theoretically ablates at the same rate as cornea, thereby leaving a perfectly smooth corneal surface with the same curvature as the lens. With this technique, there is potential to tailor the postoperative refractive effect based on the base curve of the lens selected and the resulting keratometric profile.